Microfiche readers commonly employ a folded projection [the light path is reflected by mirror(s)] onto a front or rear projection screen which is treated to intensify the image so long as the viewer is quite close to the projection axis. These readers are satisfactory for use over a short period of time but are subject to numerous problems if used for a long period. If used in high ambient light the image is washed out and lacks contrast causing eye strain. In low ambient light the contrast is too high and causes eye strain. The intensifying screen causes the image to be grainy with loss of definition and scintillation of the image . . . again causing eye strain. Since the proper viewing position is highly confined . . . when viewed too far off the projection axis there is a severe loss of brightness . . . the user is constrained to a fixed position and experiences physical discomfort. And a little appreciated factor is that the image is viewed at close range . . . say 18" . . . and requires continuous toe-in of the eyes (as does normal reading) which is fatiguing. In normal reading the reader relieves the toe-in tension by looking up but with a microfiche reader the sheer size makes it less likely the eyes will focus on something else to relieve the toe-in. Since microfiche readers deal with high magnifications at short projection distances the image brightness falls off from the center to edge and this is accentuated by the image brightening screen. This also leads to eye fatigue.
U.S. Pat. Nos. 2,562,297 and 3,893,759 show microfiche readers which project the image onto an overhead screen and the viewer looks into a small mirror close to the projection path to see the reflected image of the projected image. The reflected image will appear to be straight ahead. This image, called the virtual image, is about normal size and distance for microfiche readers. Both patents employ intensifying screens and require a confined viewing position. While small skirts hang down at the sides of the screen, the screen will be adversely affected by ambient light.
The problem of image degradation by ambient light was addressed in connection with slide and moving pictures projection in U.S. Pat. Nos. 1,624,816 and 2,015,665. The former shows a vertical box having a projector projecting an image by a folded light path onto a screen on the top inside surface. The image is viewed through an aperture in the lower front of the box by means of a mirror fixed in the box. The other patent shows a similar arrangement but projects horizontally instead of vertically. Both systems require a fairly confined viewing position due to the fixed angles of the mirrors. Both are subject to contrast degradation due to inadequate precautions against ambient light being reflected onto the screen.